1. Field of the Invention
The present invention generally relates to a wireless access method and a transmission method in a wireless communication system and, more particularly, to a communication system that enables communication between a user terminal and a network access point without interference.
2. Discussion of Background
In previous wireless access systems, as typified by a wireless LAN, a user terminal connects to an access point of a wired network so that it can get Internet services.
FIG. 1 shows a system configuration example of a wireless access system. User terminals 108a, 108b, 108c can connect to a provider network 105 through their access points 107a, 107b, 107c and via an IP router 106. Because the provider network 105 connects to the Internet 103 through a gateway equipment 104, the user terminals 108a, 108b, 108c can access a Web server 102 of a content provider 101 on the Internet 103 and download content. The access points 107a, 107b, 107c are wired to the IP router 106 by Asymmetric Digital Subscriber Lines (ADSLs) or optical fibers.
In such a wireless access system, users often buy and set up their access points and, in most cases, they do so without considerations that communication interference may occur when multiple access points use a same communication channel.
FIG. 2 is a schematic diagram that explains an access method addressing the interference problem between access points, specified in Media Access Control (MAC) sublayer specifications of the IEEE 802.11 standard for wireless LAN specifications.
A communication access method for performing data communication by random access is used, wherein a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) approach is performed which avoids collision by carrier sense so that data packets from communication nodes do not collide. A transmitting node transmits data packets after judging whether the communication channel over which to transmit data is idle by carrier sense. The carrier sense method is such that a node transmits random pulses within a given period (collision avoidance window) before transmitting data packets. When a node is not transmitting pulses, it monitors the transmission channel and transmits data packets unless pulses other than those transmitted from the node are detected. When the node detects the pulses other than those transmitted from it, it stops the transmission of data packets, waits for a given period which is randomly selected, and performs carrier sense again.
As shown in FIG. 2, while an access point 107b is transmitting data packets to a user terminal 108b, another access point 107c knows that another node is now communicating with a terminal by carrier sense. Then, the access point 107b waits for a given period and performs carrier sense again, and, after making sure that the transmission channel has become idle, transmits data packets. In this way, arrangement is provided so that data is transmitted in a time sharing manner, thus avoiding communication interference between access points.
The carrier sense method applied in the previous wireless access system avoids interference by sharing time for communication, as described above. However, a problem of this method is that data quantity that can be transmitted per unit time does not increase and, therefore, even if the number of access points increases, the total system throughput does not increase and, on the contrary, throughput per access point decreases.
FIG. 3 is a schematic diagram that explains a method for enhancing throughput by parallel transmission from access points. While another access point exists within the range of interference of one access point in FIG. 2, interference can be avoided by putting another access point out of the range of interference of one access point. Specifically, by attenuating the transmitting power of an access point and setting its transmit/receive antenna directed toward a target user terminal, radio waves from the access point does not arrive at its neighboring access point. Thus, as shown in FIG. 3, while an access point 107a is transmitting data packets to a user terminal 108a, another access point 107b can transmit data packets to another user terminal 108b. If each access point has an independent transmission channel to a user terminal, transmission capacity increases and the total system throughput increases. However, equipping a user terminal with a directional antenna and a power control function is costly because of high functionality. Therefore, such a system exists in which access points are equipped with a directional antenna and/or a power control function and user terminals are equipped with a non-directional antenna and without the power control function, and uplink transmit/receive power and downlink transmit/receive power are asymmetric. This system is, in short, such that access points can communicate with a user terminal of choice, but user terminals do not have such selectivity. As functionality of access points, for example, an access point is provided with a function to restrict the range of arrival of radio waves it transmits. As a typical example, a directional antenna, a transmission power control function, or combination thereof may be available. It is sufficient for user terminals to have a normal transmission function (non-directional antenna).
In theory, the access point function and the user terminal function are interchangeable. However, in practice equipping a user terminal with a directional antenna is quite difficult from a technical perspective.
FIG. 4 is a schematic diagram that shows a problem associated with a wireless communication system where access points are equipped with a directional antenna and power control function, where user terminals are equipped with a non-directional antenna and without the power control function. When an access point 107a is transmitting data packets to a user terminal 108a, another user terminal 108b cannot detect radio waves from the access point 107a even when it performs carrier sense and, therefore, transmits data packets to its correspondent access point 107b. At this time, at the user terminal 108a, a data packet transmitted from the access point 107a collides with a data packet transmitted from the user terminal 108b. Accordingly, a Carrier to Interference Ratio (CIR) required for decoding cannot be satisfied. Consequently, the data packets are lost. Because of no Ack reply to the transmitted data packet, the access point 107a retransmits a data packet. Such loss of packets is known as a hidden terminal problem. As countermeasures against this problem, a method for avoiding packet collision by virtual carrier sense has been proposed.
FIG. 5 is schematic diagram that shows the method for avoiding packet collision by virtual carrier sense. Immediately before transmitting data to a user terminal 108a, an access point 107a transmits a data packet called Request to Send (RTS) in which scheduled time during which it will use the transmission channel is specified. Upon having received the RTS control packet, the user terminal 108a transmits a control packet called Clear to Send (CTS) in which the scheduled time during which the transmission channel will be used is specified. When another user terminal 108b receives the CTS control packet, transmission from the user terminal 108b is prohibited for the scheduled time during which the access point 107a uses the transmission channel and data packet collision is avoided. By this method, data transmission is performed in a time sharing manner, as is the case for the carrier sense method shown in FIG. 2. Consequently, packet collision can be avoided. Unfortunately, the problem that the total system throughput does not increase even if the number of access points increases is not solved.
FIG. 6 is a schematic diagram that shows the reason why throughput does not increase even with virtual carrier sense. In the system where access points are equipped with a directional antenna and power control function, where user terminals are equipped with a non-directional antenna and without the power control function, and uplink transmit/receive power and downlink transmit/receive power are asymmetric, assume that concurrent transmission of downlink traffic from the access points to the user terminals is performed.
First, virtual carrier sense is performed by exchanging RTS and CTS control packets between an access point 107a and a user terminal 108 in order that the access point 107a transmits data to the user terminal 108a. Upon the reception of the CTS control packet from the user terminal 108a, another user terminal 108b is set in a transmit prohibition state. Then, another access point 107b transmits an RTS control packet to the user terminal 108b for data transmission thereto. The user terminal 108b receives the RTS control packet, but cannot transmit back a CTS control packet because of its transmit prohibition state. The access point 107b retries the RTS packet transmission until it receives a CTS control packet from the user terminal 108b or up to the predetermined maximum number of times of RTS retransmission. Because the communication between the access point 107b and the user terminal 108b is enabled just after the user terminal 108b is released from the transmit prohibition state, the access point 107a and the access point 107b cannot perform transmission concurrently because of the RTS and CTS control.